Review of 18 publications before the widespread use of cardiac care units disclosed that the frequency of cardiac rupture among necropsy cases of acute myocardial ranged from-4 to 24% (mean 8%) (619 of 7905 cases). We analyzed the frequency of cardiac rupture among patients studied at necropsy in our laboratory since 1968 fatal acute myocardial infarction. Of 648 such patients, 222 (34%) had cardiac rupture (left ventricular free wall, ventricular septum and/or papillary muscle). Rupture occurred in 187 (43%) of 432 patients without healed myocardial infarcts (grossly visible left ventricular scars), and in 35 (16%) of 216 patients with a healed myocardial infarct (P less than .0l). Thus, the frequency of cardiac rupture during acute myocardial infarction appears to have increased substantially since the widespread use of coronary care units, and the frequency of rupture is nearly 3 times greater in those in whom rupture occurred during the first acute myocardial infarction compared to those with a previous infarct which healed.